<!DOCTYPE html>
<html>
 <head>
   <meta charset="UTF-8">
   <title></title>
   <script>
     function $$(id){
       return document.getElementById(id);
     }
     window.onload=function(){
       var cnv=$$("canvas");
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       cxt.beginPath();
       cxt.arc(120,80,50,0,360*Math.PI/180,true);
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 <body>
   <canvas id="canvas" width="200" height="150" style="border: 1px dashed gray;"></canvas>
 </body>
</html>

<!DOCTYPE html>
<html>
 <head>
   <meta charset="UTF-8">
   <title></title>
   <script>
     function $$(id){
       return document.getElementById(id);
     }
     window.onload=function(){
       var cnv=$$("canvas");
       var cxt=cnv.getContext("2d");

       
       cxt.beginPath();
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 </head>
 <body>
   <canvas id="canvas" width="200" height="150" style="border: 1px dashed gray;"></canvas>
 </body>
</html>